i
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The latin letter is used in mathematics to define complex numbers, where represents the square root of negative one. The symbol is also used in computing and linear algebra as an index variable.
The capital Latin letter I is used in math to represent the identity matrix.
Complex numbers are an extension of the real number system with useful properties that model two dimensional space and trigonometry.
A variable is a core concept in algebra where a symbol, usualy a lower case latin letter, is used as a placeholder in a math expression.